1. Prospective study of EGFR intron 1 CA tandem repeats as predictive factor of benefit from cetuximab and irinotecan Antoniotti C 1, 2, Loupakis F 3, Cremolini C 1, 2, Zhang W 3, Yang D 3,Wakatsuki T 3, Schirripa M 1, 2, Salvatore L 1, 2, Ricci V 4, Graziano F 5, Masi G 1,2, Ruzzo A 6, LaBonte MJ 3, Ning Y 3, El-Khoueiry R 3, Falcone A 1, 2, Lenz HJ 3 1. U.O. Oncologia Medica 2 Universitaria, Azienda Ospedaliera-Universitaria Pisana, Pisa, Italy; 2. Istituto Toscano Tumori (ITT), Italy; 3. Department of Medical Oncology, University of Southern California/Norris Comprehensive Cancer Center, Los Angeles, CA; 4. Unità di Oncologia, Istituto Scientifico San Raffaele, Milano, Italy; 5. Medical Oncology Unit, Hospital of Pesaro, Pesaro, Italy; 6. Section of Biochemistry and Molecular Biology, Department of Biomolecular Sciences, University of Urbino, Urbino, Italy Abstract ID: 3540 Preclinical data suggest that the number of CA tandem repeats in EGFR intron 1 might affect gene transcription. Different retrospective studies have investigated the impact of the number of EGFR intron 1 CA repeats on the efficacy of cetuximab in mCRC patients, providing contrasting results. As reported below, different cut-off values were adopted to define polymorphic variants and patients with heterogeneous clinical and molecular characteristics were included. Results At a median f-up of 21.9 mos, mPFS and mOS were 5.2 and 13.4 mos, respectively. EGFR (CA) n genotype was L- and SS in 45 (40%) and 68( 60%) pts respectively. Ten (22%) out of 45 L- pts achieved response compared to 22 (33%) out of 67 SS pts (Fisher’s Exact test: p=0.617). No differences in PFS or OS were observed between L- and SS genotypes mPFS: 4.4 versus 5.3 mos, HR: 1.00 [95%CI: 0.67-1.51], p=0.991; mOS: 11.3 versus 14.2 mos, HR: 1.30 [95%CI: 0.80-2.22], p=0.261. Conclusions This prospective study, including a clinically homogenous and molecularly selected population, does not confirm any predictive or prognostic effect for EGFR (CA)n repeat allelic variants with respect to the efficacy of cetuximab and irinotecan in advanced lines of treatment. The present experience strengthens the need of prospectively challenging retrospective findings, to validate apparently promising biomarkers. Background N Setting Definition of allelic variantsFinding Lurje et al, Clin Can Res 2008 117Advanced lines Short : <20 CA repeats Long: ≥ 20 CA repeats No association with clinical outcome. Graziano et al, J Clin Oncol 2008 110Advanced lines Short: <17 CA repeats Long: ≥17 CA repeats SS variant is associated with longer OS Pander et al, Eur J Cancer 2010 123First-line Short: <20 CA repeats Long: ≥ 20 CA repeats L- variants are associated with shorter PFS Dahan et al, BMC Cancer 2011 56 First and subsequent lines Short:sum of CA repeats ≤35 Long:sum of CA repeats >35 No association with clinical outcome. Park et al, Canc Chem Pharm 2011 65 First and subsequent lines Short:sum of CA repeats ≤36 Long:sum of CA repeats ≥37 No association with clinical outcome. Patients and Methods We designed a prospective confirmatory study in KRAS and BRAF wild-type irinotecan-resistant mCRC patients treated with biweekly cetuximab and irinotecan in advanced lines. We defined S and L allelic variants those presenting < and ≥ 20 CA repeats, respectively. To detect a HR for PFS of 1.75 for L- compared to SS genotypes, estimating a prevalence of 60% of SS variant and setting a two-sided alfa=0.05 with a power of 80%, 104 events were required. MAIN PATIENTS’ CHARACTERISTICS Other exploratory analyses adopting different cut-off values reported in literature led to similar results. carlottantoniotti@gmail.com CharacteristicsN=113% Sex Females3632 Males7768 Age ≤65 years5650 >65 years5750 ECOG PS 06557 1-24843 N.metastatic sites 12724 >18676 Liver-only mets Yes2119 No9281